There have been four times as many cryptosporidiosis cases in the Hunter New England Health area in the first three quarters of 2009 compared with the same period in 2008.

Giardia cases have also increased by more than one-third in the same time.

While gastroenteritis cases peak in the winter, one week into summer the public pools are becoming popular and many, such as Balance Health Club at Mayfield, carry reminders to swimmers that they should shower before diving in.

Hunter New England Health Public Health physician David Durrheim said much of the increase in reported cases could be attributed to better testing for the bugs.

However, he said it was good practice to stay away from pools and spas for two weeks after stomach upsets and for people, especially children, to shower before swimming.

As well, anyone with open skin wounds such as boils should avoid the water.

Giardia and cryptosporidium are spread when people ingest the parasites, which are found in the faeces of infected humans and animals.

"People think that if they are chlorinating their pool they are killing all the bugs, but it may not be the case," Dr Durrheim said.

"Both parasites are quite hardy and can survive outside the body for a period of time, even months."

He said the stomach bugs were rarely found in public swimming pools, and none recently in the Hunter.

There was more likely to be an outbreak in child-care centres and aged-care homes but also among overseas travellers and in rainwater tanks.

"What we would do is encourage parents who have children with diarrhoea to keep them home until they are well," he said.